1. Field of the Invention
The present invention relates to an indicator composition for use in spectrophotometric detection of a substance in a solution, and a method for making the composition. In particular, the invention relates to an indicator composition formed by covalently bonding an indicator to an organohalide. The indicator composition is also an indicator and is a better indicator than the indicator from which it is made, because, when it interacts with the substance of interest, better spectral resolution is obtained. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Optical indicators are used to detect the presence and concentration of chemical compounds in solutions. Useful indicators are sensitive to the particular substance being measured, but are unaffected by the fluid and other chemical species that may be present in the fluid. A large number of optical indicators are known, offering a wide range of choices for the detection and analysis of their corresponding analytes. As used herein, the terms "analyte," "analyte of interest" and "substance of interest" mean a substance whose presence and whose concentration is the focus of investigation. The terms "indicator" and "optical indicator" mean compounds that interact with the substance in such a way as to have different optical characteristics compared to the indicator in the absence of the substance. Examples of analytes that can be detected with optical indicators include oxygen, carbon dioxide, hydrogen ion concentration (pH), metals and metal ions, oxidation-reduction potential, electrolytes, glucose, and organic compounds (gasoline, benzene, trichloroethylene, toluene, xylene, pesticides, etc.).
Indicators are used in several ways. For example, fluorescence and absorbance indicators may be mixed with a sample of the solution to be tested. Then, the difference intensity between the light incident on the sample and light transmitted through, reflected, or emitted from the sample is measured. Measurements may be made at a single frequency, several frequencies, or a range of frequencies to obtain a spectrum. The difference between the measurements represents the interaction between the indicator and the analyte or group of analytes present in the sample. Both fluorescence indicators and absorbance indicators form a chemical complex with the analyte, resulting in a color change and a shift in the fluorescence or absorption spectrum of the sample. The term "absorption" as used herein means the decrease in the intensity of light passing through a fluid sample as the result of the interaction of the incident light and the sample. Typically, a single indicator is used to measure a single analyte, however, Luebbers, et al. (U.S. Pat. No. 4,511,660) make two, sequential measurements of the pH of a solution using two different indicators, then use the differences in measured pH to generate a signal related to the ion concentration of the solution.
One of the more recent uses of indicators is with optical sensing devices. An indicator, often in combination with a sample-permeable matrix, is positioned to interact with a test sample or process stream. Light is transmitted to the indicator by an optical fiber; the interaction between the indicator and the substance to be detected alters the light transmitted through the sample prior to its receipt by a receiving fiber. The indicator-analyte complex may absorb, reflect, refract, scatter, or fluoresce in response to the incident light. The concentration of the substance can be determined by comparing the received light to the transmitted light.
Alternatively, a substrate such as paper or glass can be coated with the indicator and then placed in contact with the solution to be tested. Indicators may be incorporated into a glass or polymer matrix to form an insoluble, re-usable composite, such as those described in the following commonly-assigned patent applications: "Optical Apparatus and Method For Sensing Uranyl" Ser. No. 08/189,823, filed Feb. 1, 1994; "Tetraethyl Orthosilicate-Based Glass Composition and Method" (Ser. No. 07/999,338, filed Dec. 31 1992). These so-called "bound indicators" are in an insoluble form, and are therefore more useful for industrial and laboratory applications because they can be used repeatedly. In many applications, the indicator is placed on or near the surface of an optical fiber, and the interaction between the indicator and the solution is monitored via the optical signals carried by the fiber to a detector.
Optical indicators are used to measure the uranium concentration of process solutions in facilities for extracting uranium from ores, production of nuclear fuels, and reprocessing of irradiated fuels. For example, Fitoussi, et al. (U.S. Pat. No. 4,349,350) determine U(VI) concentration in an organic solvent from the optical density of a mixed U(VI)-dialkyl dithiophosphoric acid-organophosphorus compound complex. Volesky, et al. (U.S. Pat. No. 4,320,093) use a microbe (Rhizopus arrhizus) to separate uranium from a solution and measure uranium concentration by using arsenazo III as a color developing agent for chelate complexes of U(IV). Jungreis, et al. (U.S. Pat. No. 3,403,004) react p-dimethylaminoaniline hydrochloride with salicylaldehyde, then react the reaction product with ammonia to produce a reagent that will complex with U(VI) to form (UO.sub.2 C1.sub.4).sup.-2. Mason, et al. (U.S. Pat. No. 3,099,537) treat organic solvents containing uranium with a colorimetric agent (ammonium thioglycollate).
Presently-available absorbance indicators, including uranium-sensitive absorbance indicators, frequently have limited sensitivities due to poor resolution between the absorbance spectra of the indicator and the chemical complex formed by the indicator and the substance or analyte to be detected. In addition, spectral analysis may be complicated by the presence of other compounds that complex with the indicator and interfere with the analysis. A satisfactory indicator composition should be easily prepared, chemically stable, sensitive to low concentrations of the substance of interest, and have short response time and good separation between the absorbance spectra of the indicator and the indicator-analyte complex.